Cartilage degradation is a hallmark of two disease groups: osteoarthritis, a degenerative condition, and rheumatoid arthritis, which is primarily caused by inflammation. The degradation leads to joint pain and mobility impairment to a degree that can be considerably disabling.
Over the last two decades, considerable progress have been made in the development of anti-inflammatory agents effective for inhibiting the progress of disease. But where degradation has already taken place, new therapies are needed to assist in the regenerating of joint cartilage.
Recent efforts in the art of regenerative medicine have been directed at developing cell populations capable of repairing cartilage. International patent publication WO 96/18728 reports established lines of articular chondrocytes. WO 98/55594 reports methods for chondrocyte growth and differentiation. WO 00/27996 reports serum-free medium for chondrocyte like cells, comprising minimum essential medium, growth factors, lipids and amino acids. U.S. Pat. No. 6,150,163 (Genzyme) outlines chondrocyte media formulations and culture procedures, in which de-differentiated human articular chondrocytes are grown in a medium containing TGFβ and either insulin or. insulin-like growth factor.
Jorgensen et al. (Ann. Rheum. Dis. 60:305, 2001) reviews recent progress in stem cells for repair of cartilage and bone in arthritis. Jakob et al. (J. Cell. Biochem. 26:81, 2001) studied specific growth factors involved in expansion and redifferentiation of adult human articular chondrocytes that enhance chondrogenesis and cartilage formation. M. Brittberg (Clin. Orthop. 367 Suppl:S147, 1999) reviews current chondrocyte transplantation procedures in which pure chondrocytes or other mesenchymal cells are harvested autologously or as allografts from a healthy tissue source, expanded in vitro, and then implanted into the defect at high density.
Despite the initial success of these clinical methods, it is clear that current sources of chondrocytes are inadequate to treat most of the instances of cartilage degeneration that present themselves at the clinic.
Kramer et al. (Mech. Dev. 92:193, 2000) reported that mouse embryonic stem cells can be modulated with bone morphogenic proteins (BMP-2 and BMP-4) to produce cells that stained with Alcian blue, a feature of chondrocytes, and expressing the transcription factor scleraxis. Regrettably, the mouse model of embryonic stem cell development is its own peculiar case, and does not yield strategies for differentiation that are applicable to other species. In fact, there are very few other mammalian species from which pluripotent stem cells have been reproducibly isolated.
Only recently did Thomson et al. isolate embryonic stem cells from human blastocysts (Science 282:114, 1998). Concurrently, Gearhart and coworkers derived human embryonic germ (hEG) cell lines from fetal gonadal tissue (Shamblott et al., Proc. Natl. Acad. Sci. USA 95:13726, 1998). Unlike mouse embryonic stem cells, which can be kept from differentiation simply by culturing with Leukemia Inhibitory Factor (LIF), human embryonic stem cells must be maintained under very special conditions (U.S. Pat. No. 6,200,806; WO 99/20741; WO 01/51616).
Accordingly, it is necessary to develop whole new paradigms to differentiate human pluripotent cells into fully functional differentiated cell types.